Oscars....

[TPS reports]

a tank that size is great to grow out your aro.
once he hits over a foot you'll want something bigger though.
you can keep several smaller fish in there while he's younger...but everything will slowly get eaten as he grows bigger.

as for oscars, you can easily keep two in there given that they are under a foot or so.
once they hit around a foot and over, you'll need a bigger tank.
if you just kept one oscar you could keep smaller cichlids in with it as well, they might or might not get eaten.

 

[enervatedlight]

Oscars are real passive aggressive..
hypocritical little MFK'ers (hint: they don't take care of fish)

My Oscar used to get along with all the fish I had with it...red tailed tetras, bolivian rams, etc.

One day...it goes apesh*t on my fish and the tetras start disappearing...one ram dies....and the other ram has bite marks on its fins.

I introduce it to my aro tank, with the silver arowana being around 8" and the oscar being 4-5" at most. It's all good and well for about a week....

I turn on the light one day and the the aro is missing a chunk out of his side and has split fins....I remove the oscar....and lo and behold the aro is like new.

If I were you, based off of MY experience, I would NOT put the oscar with the aro.

 

[TPS reports]

sounds like you had a mean ass oscar.